With the increasing popularity of wide area networks, such as the Internet and/or World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions is the use of wireless optical telecommunications technology. Wireless optical telecommunication systems, also known as “free-space optical” (FSO) communication systems, utilize beams of light, such as lasers, as optical communications signals, and therefore do not require the routing of cables or fibers between locations. Data or information is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver.
For point-to-point free space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of a transmitter and of the receiver properly pointed at each other. For example, a transmitted optical beam with a 1-mrad divergence has a spot diameter at the receiver of about 1 meter at a 1-km range. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field-of-view) could compromise the link unless active tracking is employed. Since high-speed communication channels utilize extremely sensitive detectors, such systems require equally sensitive tracking systems.
Charge coupled device (CCD) arrays or quadrant cell optical detectors (sometimes referred to as “quad cells”) may be used as tracking detectors in a tracking system. In either case, an electrically controllable steering mirror, gimbal, or other steering device may be used to maximize an optical signal (e.g., light) directed at a high-speed detector, based on information provided by the tracking detector. This is possible since optical paths for tracking and communication are pre-aligned, and the nature of a tracking signal for a perfectly aligned system is known.
A schematic diagram corresponding to a typical optic position correction control loop used in a FSO transceiver 310 is illustrated in FIG. 1. The objective of the control loop is to control the position of a fast steering mirror 312 such that a maximum optical signal is received by a receive path 313. In the illustrated configuration, incoming light comprising a received optical signal 314 is received by a telescope 316 including a plurality of lenses (not shown), which collimates the optical signal into a collimated beam 318. The collimated beam is directed toward fast steering mirror 312, which redirects the light toward a beam splitter 320. The beam splitter directs a majority (e.g., 80-90%) of the beam's energy toward a lens 322 that converges the light toward its focal point, which coincides with receive path 312, whereupon the received signal is processed by a signal processing block 324 to generate data 326. A remaining portion (e.g., 10-20%) of the beam's energy passes through the beam splitter and is received at lens 328, which focuses the light toward a beam position sensor 330 that is located coincident to the lens' focal point. Generally, the beam position sensor may comprise a quad cell, CCD (charge-coupled device), electronic camera, or any other sensor that is capable of detecting the position of a light beam. The beam position sensor generates an two-axis position error signal (or position data from which an error signal can be derived), which is received by a position controller 332. The position controller processes the two-axis position error signal or position data to generate a two-axis torque command signal that is used to drive a two-axis mirror driver 334 coupled to the fast steering mirror. Based on the beam position sensor's output, the closed loop control system drives the position of the fast steering mirror such that the optical signal is directed toward beam splitter 320 in an manner that optimizes the optical signal received by receive path 313 using conventional closed-loop feedback, which typically is used to position either the entire FSO transceiver or optical components included therein, such as steering mirror 312.
In order to maintain an optimal signal received by the receive path, it is necessary that the fast steering mirror be able to compensate for various mechanical disturbances imposed on the FSO transceiver, such as building movement and vibrations. Preferably, the bandwidth of the fast steering mirror positioner should be 5-10 times greater than that of the mechanical disturbances.